The present invention relates to a method of manufacturing an integrated thin film solar battery module having a plurality of unit cells formed on a substrate, in which defects generated in the step of partially removing a second electrode for dividing the second electrode corresponding to unit cells are eliminated and the contact interface between a semiconductor layer and the second electrode is improved so as to contribute to the development of an integrated thin film solar battery having a high conversion efficiency.
In recent years, a solar battery in which energy of the solar light is converted directly into electric energy has begun to be used widely. As a matter of fact, a crystalline solar battery using a single crystalline silicon or a polycrystalline silicon has already been put to a practical use as an outdoor solar battery for generating an electric power. On the other hand, a thin film solar battery using an amorphous silicon, etc., which permits decreasing the raw materials and, thus, attracts attentions as a low cost solar battery, is presently on the stage of development as a whole. Vigorous studies are being made nowadays on the thin film solar battery in an attempt to permit the solar battery to be used outdoors on the basis of the actual results achieved in the use as a power source for civil electric appliances such as hand-held calculators that have already been put widely to a practical use.
Deposition of a thin film by means of CVD, sputtering, etc., and patterning of the deposited thin film are repeatedly carried out for forming a desired structure of a thin film solar battery, as in the manufacture of the conventional thin film device. In general, employed is an integrated structure in which a plurality of unit cells are connected in series on a single substrate. When it comes to a solar battery arranged outdoors for power generation, the solar battery includes a substrate having a very large area exceeding, for example, 400.times.800 (mm).
FIG. 1 is a cross sectional view showing the construction of a thin film solar battery. FIG. 2 is a plan view schematically showing the thin film solar battery shown in FIG. 1. As shown in the drawings, a first electrode layer 2, a semiconductor layer 4 consisting of, for example, an amorphous silicon, and a second electrode layer 6 are laminated one upon the other in the order mentioned on a glass substrate 1. These layers are divided corresponding to a plurality of unit cells 11. The second electrode layer 6 and the first electrode layer 2 are connected to each other via openings for connection, i.e., scribe lines 5 formed in the semiconductor layer 4, and the adjacent unit cells 11 are connected in series.
The first electrode layer 2 consists of a transparent conductive oxide such as tin oxide (SnO.sub.2), zinc oxide (ZnO), or indium tin oxide (ITO). On the other hand, the second electrode layer 6 consists of a metal film formed of aluminum (Al), silver (Ag), or chromium (Cr).
The integrated thin film solar battery of the particular construction is prepared as follows. In the first step, a transparent conductive oxide such as SnO.sub.2, ZnO or ITO is deposited on the glass substrate 1 to form the first electrode layer 2. The first electrode layer 2 thus formed is laser-scribed at the positions of the scribe lines 3 to divide the first electrode layer 2 corresponding to the plural unit cells (power generating regions). The substrate is washed for removing the molten residue generated by laser-scribing. Then, a semiconductor layer 4 made of amorphous silicon and having a pin junction structure is deposited by a plasma CVD method. The semiconductor layer 4 thus formed is partly laser-scribed at positions of the scribe lines 5 about 100 .mu.m away from the scribe lines 3 of the first electrode layer 2. The scribe line 5 provides an opening for connection of the second electrode layer and the first electrode layer. Then, a metal film consisting of, for example, Al, Ag or Cr is formed as the second electrode layer 6 in the form of a single layer or a plurality of layers on the semiconductor layer 4. The second electrode layer 6 thus formed is partly laser-scribed at positions of scribe lines 7 about 100 .mu.m away from the scribe lines 5 of the semiconductor layer 4. In this step, the second electrode layer 6 and the semiconductor layer 4 positioned below the layer 6 are successively removed at the positions of the scribe lines 7. Thus, an integrated thin film solar battery having a plurality of unit cells connected in series is fabricated.
In the next step, a filler made of a thermosetting resin such as ethylene-vinyl acetate copolymer (EVA) and a protective film consisting of, for example, fluorocarbon resin, e.g., Tedler manufactured by Du Pont, is laminated on the back surface of the thin film solar battery, followed by encapsulating by means of, for example, a vacuum laminator. Then, a frame is mounted to surround the thin film solar battery, thereby completing a thin film solar battery module.
The conventional integrated thin film solar battery was defective in its output characteristics. Particularly, the fill-factor (FF value) of the conventional solar battery was low. In the manufacture of an integrated thin film solar battery, it is attempted to make optimum the process conditions such as the thickness of the first and second electrode layers 2 and 6 and film quality of the semiconductor layer 4 in order to improve the characteristics of the solar battery. When it comes to a substrate having a large area, however, the experiment for making the process conditions optimum is rendered complex. Therefore, an auxiliary experiment is conducted first for preparing a thin film solar battery having a small area by a simplified process so as to evaluate the characteristics of the solar battery and to determine the optimum process conditions. The optimum conditions thus obtained are applied to the manufacturing process of a thin film solar battery having a large area.
However, where the process conditions optimum for the manufacture of a thin film solar battery having a small area are applied as they are to the manufacture of a thin film solar battery having a large area, it is difficult to obtain satisfactory results as obtained in the auxiliary experiment. In many cases, the FF values of the solar battery having a large area are lowered. Under the circumstances, it is absolutely necessary and urgently required to improve the FF value for improving the conversion efficiency in an integrated thin film solar battery having a large area.
As a result of an extensive research, the present inventor has found that the decrease in the FF value of a thin film solar battery is considered to be caused by two factors. First of all, attentions should be paid to a poor interface between the semiconductor layer 4 and the second electrode 6. This problem can be solved by forming a conductive layer on the semiconductor layer so as to prevent a native oxide film from being formed in the washing step after the scribing of the semiconductor layer, as disclosed in Japanese Patent Disclosure (Kokai) No. 9-8337. A second problem is that a short-circuit or electric conduction takes place in that portion of the semiconductor layer which is removed subsequent to the removal of a part of the second electrode layer. To be more specific, a fresh surface of the semiconductor layer appears between the second electrode layer and the first electrode layer at the position of the scribing line of the second electrode layer. Since the fresh surface of the semiconductor layer is unstable, the electric resistance is lowered, if an impurity is attached to the fresh surface even if the amount of the impurity is very small. As a result, a short-circuit or electric conduction is caused to take place between the second electrode layer and the first electrode layer.
The present inventor has studied to apply known methods in an attempt to solve the second problem described above. It has been found, however, that the known methods are incapable of overcoming the problem.
For example, Japanese Patent Disclosure No. 61-198685 discloses an idea that a semiconductor layer is scribed and divided by means of a laser beam irradiation under an oxidizing atmosphere. The present inventor has studied an idea of oxidizing the exposed surface of a semiconductor layer by applying the laser beam irradiation under an oxidizing atmosphere to the scribing of the second electrode layer. However, it is difficult to retain a substrate having a large area under an oxidizing atmosphere. In addition, heat of high temperature tends to be locally generated during the laser beam irradiation, leading to fire occurrence. In other words, the laser beam irradiation is dangerous.
Similarly, Japanese Patent Disclosure No. 61-156775 teaches that an amorphous solar battery of a stacked structure in which a metal electrode, an amorphous silicon layer and a transparent electrode are formed on a substrate is subjected to a laser processing under a heated steam atmosphere to divide the stacked structure into cells. It is also taught that the divided cells are subjected to a heat treatment at a temperature ranging from 150 to 300.degree. C. In this method, a heated steam is blown in the laser processing step to prevent the amorphous silicon layer from being crystallized. It has been found, however, that, in this method, the substrate linearly expands by heating so as to markedly lower processing accuracy with the laser beam. Also, it is difficult to remove completely a residue in the processed portion even if a steam-containing carrier gas is blown in the laser processing step. It is particularly difficult to remove the residue completely where the laser processing is performed at a high speed. In this case, it is substantially impossible to allow the processed edge surface to be exposed completely to the outside.
Japanese Patent Disclosure No. 61-280679 discloses an idea of forming an insulating layer thicker than the semiconductor layer in that region of the first electrode layer which corresponds to the scribe line of the second electrode layer. In this case, since the insulating layer is formed below the semiconductor layer, a cross section of the semiconductor layer does not appear between the first electrode layer and the second electrode layer when the second electrode layer and the semiconductor layer below the second electrode layer are scribed. However, formation of the insulating layer decreases the area of the cells capable of power generation. In addition, it is difficult to form accurately an insulating layer to conform with cells formed on a large area substrate.
Further, Japanese Patent Disclosure No. 60-85574 discloses an idea of using Cr or Ni for forming the second electrode. The disclosure is based on an erroneous recognition that the second electrode forms an alloy with silicon in the scribing step of the second electrode so as to bring about electric conduction. However, the technical idea disclosed in this prior art is clearly incapable of solving the problem in respect of the decreased FF value.
The other technique to restore fill-factor is disclosed in U.S. Pat. No. 4,371,738, which described Staebler-Wronski effect that the defects in hydrogenated amorphous silicon induced by light irradiation annealed out by heating one. The technique can only restore the defects induced by light irradiation, but the patent does not have a concept of restoring defects induced in silicon layer during the laser scribing of the second electrode.